During hover and low-speed flight, helicopters experience significant aerodynamic forces on the tail boom caused by the wake from the main and tail rotors and by crosswinds. A helicopter is subjected to complex airflows that are self-imposed as a result of the main and tail rotor wakes and as a result of ambient wind. These airflows produce aerodynamic forces on the fuselage and tail boom assembly during hover and low speed sideward flight. When operating in this portion of the flight envelope, the airflow from the main rotor wake has a large impact on the tail boom forces. These effects cause the tail boom to be subjected to air loads which must be counteracted by main rotor and tail rotor thrust to maintain aircraft trim. These increased power requirements result in a reduction in payload and yaw control margin.
In order to provide sufficient torque control and adequate sideslip ability, most prior helicopter designs have employed a combination of tail fin and a relatively large, power consuming, tail rotor. The problems associated with the use of this type tail rotor are well documented in the prior art. The foremost problem is the hazard presented by the tail rotor, particularly to ground personnel. Additionally, in military helicopters, the tail rotor is vulnerable to combat damage which can result in catastrophic loss of control of the helicopter. Also, the reliability and maintainability problems of a tail rotor, with its gearboxes, bearings, and drive shafts, are severe. Further, during high speed flight the tail rotor requirement can be reduced, since yaw control can be provided by aerodynamic surfaces. However, the typical tail rotor continues to use excessive engine power and to produce adverse drag effects.
In order to avoid these problems, efforts have been made to eliminate tail rotors. In U.S. Pat. No. 4,200,252 (Logan) and earlier referenced patents, helicopter anti-torque systems are disclosed which employ the principles of fuselage circulation control using the main rotor downwash. These inventions describe how exhaust or other engine-driven air may be ducted into the helicopter aft fuselage section and then be injected tangential to the fuselage in order to induce more circulation. This increase in circulation is achieved by use of main rotor downwash to produce additional lateral forces on the fuselage which oppose main rotor torque.
However, it is established in prior art that main rotor downwash does not flow over the fuselage during high speed flight. The forward velocity of the helicopter moves the fuselage clear before the wash can reach it. Viewed from within the helicopter, the downwash pattern appears to have a large rearward horizontal component. Thus, a supplemental means of directional control, other than circulation control alone, is necessary. These supplemental means include tail rotors, aerodynamic fins, and reaction jets.
Two U.S. patents specifically address circulation control, Logan supra and U.S. Pat. No. 3,807,662. Both of these patents contemplate increasing the favorable lateral forces on the helicopter fuselage. These types of devices result in increased complexity and weight. The ducting, plenum and nozzle arrangements require considerable redesign and modification of the helicopter. Further, the possibility of mechanical failure and the increased vulnerability in the case of a military helicopter remain inherent deficiencies.
U.S. Pat. No. 4,708,305 partially solves the low-speed yaw control problem by placing two strakes on the side of the fuselage facing the approaching main rotor blade. This arrangement results in a significant download on the tail boom and is only beneficial in reducing side thrust and yaw control in right sideward flight.
Accordingly, it is an object of the present invention to provide upper and lower fuselage strakes located on opposite sides of the helicopter fuselage which will beneficially alter the air flow around the helicopter tail boom.
It is another object of this invention to improve left and right sideward flight.
It is another object of this invention to reduce the load requirements on the helicopter torque control means.
It is another object of this invention to reduce the size of the helicopter torque control means by using fuselage air loads to provide part of the needed torque control.
A further object of the invention is to increase helicopter sideslip ability by controlling air flow circulation around the fuselage.
Another object of the invention is to provide a retraction/extension mechanism whereby the strakes may be positioned for optimal performance.
A further object of the invention is to provide improved reliability and maintainability for the helicopter torque control means by reducing power and load requirements.
Still another object of the present invention is to improve helicopter performance through increased cross wind speed in hover, increased fuel savings and increased load capacity by decreasing power required from the torque control, by decreasing drag inherent in the torque control and by reducing weight of the torque control means.
Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.